DEAR DR. BLONZ >> A series of reactions had my family physician order allergy testing. The results were a bit surprising because they identified items I had never reacted to before. Why might I have developed an allergic reaction to a food when there was no problem for so many years?

W.S., Tulsa, Oklahoma

DEAR W.S. >> Food allergies are a more common experience in our early years, but they can develop at any age — even to foods you once tolerated and enjoyed. A true food allergy occurs when the body’s immune system is misled into believing that a normally harmless food is an invader out to cause harm. It begins when a food part, usually a protein fragment, slips through the intestinal wall into the digestive system before it’s been completely digested. After that absorption, the fragment is encountered by the immune system, our body’s security force. Because that fragment is someplace it doesn’t belong, the immune system reacts as it would to any trespasser, carrying out its primary mission to attack and eliminate gate crashers while preparing the body for future encounters.

Eating that same food later can cause the immune system to respond with symptoms like GI upset, sneezing, runny nose, asthma, skin rashes, swelling, headache and even a life-threatening drop in blood pressure. The most common food allergies involve nuts, eggs, shellfish, wheat, milk and soy. Why these? It may be that during their disassembly during digestion, these can form segments with a peculiar ability to slip through the intestinal wall. Some types of allergies developing in childhood can be outgrown, while others persist into adulthood.

Events affecting our immune system can influence the tendency to develop food allergies. Such events can include hormonal changes, medications, body stressors, infections and illnesses, particularly ones affecting digestion or the immune system. Drugs and alcohol can also have an impact. Our genetics are also involved. All these variables can make you susceptible to a food allergy reaction that might not normally occur. In addition, changes in geographical location can raise the level of allergens above your particular sensitivity threshold.

Those with allergy symptoms suspected of being food-related should consider keeping a diary. By recording all your meals and then comparing this with your reactions, you may begin to see some patterns. Always seek qualified assistance if you or a member of your family want professional help with your food allergy investigation. A good place to start is with physicians certified through the American College of Allergy and Immunology. Get more information at foodallergy.org.

Kensington resident Ed Blonz has a PhD in nutrition from UC Davis. Email him at cctimes@blonz.com.